I bought a Samsung 200GB PATA drive yesterday, and mounted it in a Mad Dog 3.5" Firewire/USB enclosure. I created one partition for the entire disk in Windows XP, and formatted the entire drive as FAT32 from OS X 10.4.5. It reported 186 GB and change free (correct), and the free space dropped correctly as I moved a number of relatively small files/directories onto the drive. Tonight, I've been creating Acronis drive images (across the network from my Windows laptops and desktop), and when I view the info on those files/folders I see a total of about 50GB taken up. Now though, the drive is reporting only about 60GB free viewed in Finder, or if I do a "get info". I can't find the approx 70GB that has "disappeared".
After keeping an eye on this as the image files are being copied over, I find that:

Each image file shows 4GB, as it should, and as is consistent with FAT32 limitations. However, when that file is completely written, the free space is dropping by what appears to be a multiple of 5! So, a 4GB file, once written to the drive, is reducing the free space shown in OS X by 20GB. Keep in mind it's Acronis and Windows machines that are actually writing to the drive across the network, but this still seems mighty odd. Can OS X not "do the math?"

Update 2, or is it 3?

I have more than enough redundancy with other external drives for my Windows box and laptops. I decided to split the 200 GB in half, making one partition extended/journaled (perhaps for some linux experimental installs), and the other partition for archival purposes for the mini. So, it's a non-issue for me now, although I'd still be interested in an explanation if anyone has one. Thanks!